Known brushless alternator systems comprising a main generator and an exciter for providing field current to the main generator include a voltage regulator for controlling the current delivered to a field winding of the exciter to in turn control the output voltage of the generator. These voltage regulators control exciter field current in accordance with the output voltage and/or output current of the main generator.
In an excessive current condition, such as a fault situation, the output voltage of the generator will drop significantly. The higher currents which are experienced in the fault condition can damage generator components, while a drop in the voltage out of the regulator can result in the loss of information stored in volatile memory components of systems being powered by the generator output. It is therefore desirable to limit current out of the generator, and at the same time maintain the excitation of the generator until the fault can be cleared.
Hucker U.S. Pat. No. 4,442,396, assigned to the assignee of the instant application, discloses a generator system which maintains excitation during a fault condition. The current limiting function is provided in the form of silicon controlled rectifiers which are "phased back" in the event of a fault so that the voltage at the output of the main generator armature can be maintained at a sufficiently high value to permit continued self-excitation.
The generating system of the Hucker application provides current limiting based on the highest phase current in a three phase generating system. This generating system does not take into account the fact that faults of different magnitude, such as three-phase shorts, single-phase to neutral shorts or two-phase to neutral shorts, require different levels of field excitation. Ideally, the voltage regulator will provide the same level of current limiting regardless of the type of fault.
In Applicants copending application, Ser. No. 648,301, a voltage regulator for a generating system controls exciter current as a function of the average of the three phase output voltage and the peak phase voltage. This voltage regulator provides a current limiting function when a three phase fault occurs, however it will not respond to effectively limit the maximum current when there is a single phase or two phase fault.
The present invention is directed to overcoming one or more of the problems as set forth above.